1. Field of the Invention
This invention generally relates to the electrode structure of a light-emitting element. More particularly, the present invention provides an electrode structure defined by continuous hexagonal structures to equalize the current density and the light-emitting intensity in the light-emitting element.
2. Description of the Prior Art
Since GaN possesses a wide band gap (Eg=3.4 eV in room temperature) and its spectrum area is near to the wavelength of blue light, GaN is a kind of suitable material for manufacturing short wavelength light-emitting devices, and it further becomes one of the most popular material for developing photoelectric elements. Nowadays, through being continually researched and developed, GaN can epitaxially grow more steadily on a sapphire. Moreover, GaN can be utilized in making short wavelength light-emitting devices as long as it passes through a suitable epitaxial growth and electrode arrangement.
Referring to FIG. 1, a top view of the schematic electrode structure of a well-known light-emitting device is illustrated. While an external voltage is applied to a first electrode 110 through a contact pad, currents flow into the light-emitting device. The currents pass forward through the internal heterogeneous junction, such as a P/N junction, and make the light-emitting device luminescent due to energy conversion resulted from minor carriers recombining. Then, the currents converge on a second electrode 120 via different current paths, such as current paths 112, 114, and 116, etc., and flow out the light-emitting device by another contact pad. However, the distances of the current paths from the first electrode 110 to the second electrode 120 are not equidistant. This situation makes the current density in the light-emitting device inconsistent and further results in different light-emitting intensity. This is because the current density is an inverse ratio to the length of the current path, and the light-emitting intensity is a direct proportion to the current density. For example, the current density on edge current paths 112 and 116 is lower than the current density on a central current path 114, since the edge current paths 112 and 116 are longer than the central current path 114. Therefore, the light-emitting intensity on the edges of the light-emitting device is not as light as it is on the center. In addition, the inconsistent current density is one of the main factors in poor reliability in the light-emitting device.
In order to increase the brightness of the light-emitting device, the GaN light-emitting devices have gradually developed toward the field of high power and big area elements. As shown in FIG. 2, a top view of a schematic finger-interlaced electrode structure is illustrated. The kind of finger-interlaced electrode structure is commonly used for the high power semiconductor elements. A first electrode 150 vertically connects to extending electrodes 150-1 and 150-2, which parallel to each other, to form a finger electrode structure. A second electrode 160 also vertically connects to extending electrodes 160-1, 160-2 and 160-3, which parallel to each other, to form another finger electrode structure. A so-called finger-interlaced electrode structure is formed through paralleling and interlacing the extending electrodes 160-1, 150-1, 160-2, 150-2, and 160-3. The finger-interlaced electrode structure makes each extending electrode have the same distance to its adjacent extending electrodes. Thus, the distances of current paths for currents flowing from any extending electrode to its adjacent extending electrodes are equidistant, so as to equalize the current density and the light-emitting intensity in a light-emitting device. However, the problem is the resistance on the extending electrode is a direct proportion to the distance to the electrode. For example, the resistance for point B to the first electrode 150 is bigger than the resistance for point A to the first electrode 150. This means the current intensity and density on point B are lower than those on point A, and hence, the light-emitting intensity around point B is weaker than around point A.
In view of the drawbacks mentioned with the prior art of electrode structures, there is a continued need to develop a new and improved structure that overcomes the disadvantages associated with the prior art of electrode structures. The advantages of this invention are that it solves the problems mentioned above.